The present invention generally relates to a developing device which uses a magnetic one-component toner, and particularly relates to a toner layer forming device for forming a thin layer of a magnetic one-component toner on a developing sleeve.
In classifying developing methods for developing an electrostatically charged pattern in view of a composition of a developer, there is a two-component developing method and a one-component developing method. In the two-component developing method, the developer is composed of a mixture of carrier particles, such as iron powder or glass-beads etc., and toner particles which are used to actually develop an electrostatic latent image. The two-component developing method has disadvantages such as fluctuation in image density caused by variations in mixture ratio between the carrier particles and the toner particles, and reduction in image quality owing to deterioration of the carrier particles.
By contrast, the one-component developing method uses no carrier particles and therefore the foregoing disadvantages of the two-component developing method are not encountered. As a result, mono-component developers have a bright future for use in developing methods. The one-component developers used in the one-component developing method are roughly classified into nonmagnetic one-component developers and magnetic one-component developers. The magnetic one-component developer includes magnetic powder in the toner particles, because of the necessity of means for carrying the developer to the developing region facing an electrostatic image.
In a conventional magnetic one-component developing method, a nonmagnetic bartrimmer faces a developing sleeve so as to form a thin layer of the toner particles on the developing sleeve while rubbing the toner particles to thereby charge the toner particles with triboelectricity. Thus, a toner image corresponding to the electrostatic latent image on a photosensitive drum is formed on the photosensitive drum when the thin layer of the toner particles comes into opposition to the photosensitive drum.
In the conventional developing device using a magnetic one-component toner, however, it is difficult to form a uniform thin layer of toner particles in the axial direction of the developing sleeve, and toner particles may be sandwiched between the developing sleeve and the nonmagnetic bartrimmer, and are apt to cause poor formation of the thin layer of the toner particles at that portion.
In a toner layer forming method using a magnetic bartrimmer (for example, the methods described in Japanese Patent Publication Nos. 33916/84 and 29873/84), sufficient charge property cannot be obtained in the case of some kinds of developers, so that high picture quality is not always attained.
Further, in a developing device using an apron blade 4 as shown in FIG. 5, the apron blade 4 is rubbed by a developing sleeve 3, so that excessive friction may occur on the developing sleeve 3 or the like. As a result, toner particles may become unevenly charged. In this developing device, there is a further problem that the developing sleeve 3 and the apron blade 4 are arranged in the form of a wedge upstream of the apron blade 4, so that the toner particles may adhere to the wedge-shaped portion.